Laser Surface Modification Process Research Articles

Hot corrosion behavior of CYSZ thermal barrier coating with optimized laser surface modification

The current study aims to investigate the influence of laser surface modification on the hot corrosion, fracture toughness, and hardness resistance of atmospheric plasma sprayed Ceria-Yttria Stabilized Zirconia (CYSZ) thermal barrier coating. Throughout the laser surface modification process, a variety of laser processing parameters are used to obtain the finest surface properties and optimum laser parameters. In this study, the whole coating surface is modified using the optimal parameters. The results show that 52.9 W laser power at a 220 mm laser distance and a 60 mm/s scanning speed were found as the optimum laser glazing parameters. A hot corrosion test is performed in a box furnace for 20 h at 1050 °C. Scanning electron microscopy (SEM) images show denser surfaces in laser-glazed coatings when compared to air-spraying coatings. A dense microstructure combined with the top coat is created in the coating cross-section, and a smooth microstructure with a coaxial fracture network is produced on the surface. The laser-glazed layer has superior mechanical characteristics to the as-sprayed coating, including hardness and fracture toughness. This layer also restricts the damaging monoclinic-tetragonal phase change for CYSZ TBC, blocking the infiltration of molten salt. The hot corrosion behavior of TBCs is improved by the inclusion of a dense laser-glazed layer.

Study on the Surface Modification of Nanostructured Ti Alloys and Coarse-Grained Ti Alloys

Commercial purity titanium (CP-Ti) and a Ti-6Al-4V alloy (Ti64) were processed by high-pressure torsion (HPT) for 10 and 20 turns. The HPT processing produced a nanostructured microstructure and a significant strength enhancement in the CP-Ti and Ti64 samples. After 20 turns, the samples of HPT-processed CP-Ti and Ti64 were subjected to laser surface treatments in an air atmosphere using different scanning speeds and laser powers. The surface roughness of the laser-modified samples increased with increasing laser power and this produced hydrophilicity due to a lower contact angle. After a holding time of 27 days, these samples underwent a hydrophilic-to-hydrophobic transformation as the contact angle increased from 13° to as much as 120° for the CP-Ti sample, and for the Ti64 sample the contact angle increased from 10° to 126°. In addition, the laser surface modification process was carried out with different atmospheres (air, vacuum and O2) on heat-treated but unstrained CP-Ti and Ti64 samples and the contact angle changed due to the surface element content. Thus, as the carbon content increased from 28% to 47% in CP-Ti in a vacuum environment, the surface contact angle increased from 22° to 140°. When a laser surface modification process is conducted under oxygen-less conditions, it is concluded that the contact angle increases rapidly in order to control the hydrophobic properties of Ti and the Ti alloy.

Enhancement of laser heating process by laser surface modification on Titanium alloy

Titanium alloys are widely utilized in laser heating applications. However, it has poor optical properties due to low laser energy absorption. Nevertheless, a higher energy absorption can be realized by modifying the surface profile through increasing the surface roughness. In this present work, the laser surface modification (LSM) process was carried out to increase the roughness on surface of Ti6Al4V titanium alloy. Subsequently, the surface characterization and surface roughness were analysed by using the 3D optical microscope. The effect of laser power on the increment of surface roughness was investigated. It was revealed that an increase in laser power during LSM process could increase the surface roughness. The result shows that, the surface roughness of titanium alloy increased 27 times when modified with the highest laser power (27W) compared to the gritted surface. Furthermore, the modified surface by LSM will be heated using laser radiation in order to analyse the effect of surface roughness towards laser heating temperature. Depending on the value of the power during laser heating, the maximum temperature measured could be increased 27% corresponding to a gritted flat reference surface.

Modeling of laser surface modification processes of tool steel to predict the temperature distribution and modification zone

Process of metal surface modification during laser treatment requires a long search and control of the treatment technological parameters, depending on the technical requirements imposed on it. The article proposes a method of mathematical modeling of heat transfer during laser processing of the high carbon steel 9HS (equivalent to DIN 150Cr14). The dimensions of the hardening area are calculated with high accuracy by the isotherm of the critical temperature Ac3. The influence of the main technological parameters (radiation power, laser spot diameter, processing speed) on the depth and width of the hardening zone is shown. The paper also presents the use of the build model for the selection of multitrack processing modes to harden high-carbon steel 9HS to the required depth without surface melting.

Laser processing of multiple surface characteristics for Ti6Al4V biomedical implants

ABSTRACT Surface roughness, microhardness, and wettability are the surface characteristics which significantly influence the implant performance. Laser surface modification with raster scan is a well-known technique for processing the material surfaces. This technique has potential to modify multiple surface characteristics together. However, multiple process parameters are involved and each parameter can affect individual surface characteristics differently. This work focuses on processing multiple surface characteristics (surface roughness, micro-hardness, and wettability) with laser surface modification. Laser surface modification of Ti6Al4V is performed with wide range of process parameters. The laser modified samples are characterized, and surface roughness, microhardness, and wettability are measured. A modified heat input parameter is introduced, which accounts for multiple LSM process parameters. The range of the modified heat input parameter for various implants are identified, by superimposing the modified heat input parameters for experiments against the surface requirements of implants, on the Ashby styled three-dimensional projection plot. Comparable combinations of surface characteristics are achieved from different combinations of the process parameters with similar modified heat input parameters. This study shows that multiple surface characteristics can be processed together to meet implant specific requirements and full potential of the laser surface modification process can be utilized.

Ti6Al4V functionally graded material via high power and high speed laser surface modification

This study investigates the fabrication of Ti6Al4V functionally graded material via high power and high speed laser surface modification (LSM). The original sample microstructures consisted of elongated equiaxed α phase with a grain boundary of β phase. Nine different LSM process parameter sets were applied to these samples. Scanning electron microscopy showed a fine acicular martensitic phase next to the surface of the laser treated samples in all cases. A transition microstructure zone beneath the martensitic zone was observed, with larger, equiaxed grains and some martensitic α phase growth. The interior of the sample contained the original microstructure. The surface roughness was found to increase after the surface modification for all process parameter sets. Nanoindentation tests were performed in order to obtain the hardness and modulus of the three phases, i.e. martensitic α, equiaxed α and the grain boundary β. A dual phase crystal plasticity finite element model was developed to investigate the three zone functionally graded microstructure under uniaxial tensile loading. The hardened surface zone prevented the propagation of continuous slip bands, while the transition zone prevented excessively sharp stress concentrations between the outer surface and interior of the samples.

Effect of Laser Surface Modification (LSM) on laser energy absorption for laser brazing

Since the development of the laser in the 1960s a rapid development of research interests in science and technology took place. Since then, the need of laser application in industrials such as automotive, aerospace and electronics is increasing because of several advantages like automation worthiness, noncontact processing and product quality improvement. In this present study, the effect of Laser Surface Modification (LSM) on pure copper plate towards the laser energy absorption during indirect laser brazing process was studied. The laser brazing experiment was conducted inside a chamber under controlled vacuum pressure with 400Pa and irradiated with constant 140 Watt laser power. The defocusing features for laser brazing was used in order to find better focal position. Accordingly, the focal length for this laser brazing experiment was set to the focus point at 124 mm from the focal plane. Meanwhile, during LSM process, laser parameters such as laser scanning speed and focus length have been kept constant throughout the surface modification process. Yet, the laser power and laser frequency have been varied from 9 Watt to 27 Watt and 10 kHz to 80 kHz respectively. Apparently, surface roughness due to surface removal and oxide layer formation were presented during LSM process. These two surface integrities were found to be the factors of increasing laser energy absorption. It was discovered that an increase in surface roughness and oxide layer formation can absorb more laser energy which then results an increase in brazing temperature during laser brazing. This is because, increasing surface roughness will scatter the laser energy over a larger surface area, multiply the reflections in the surface irregularities while the oxide layer will enhance the interference phenomena of laser energy occurring inside the oxide layer. Both mechanisms increase laser energy absorptivity during laser brazing which results a high brazing temperature.

Flash Surface Treatment of CH3NH3PbI3 Films Using 248 nm KrF Excimer Laser Enhances the Performance of Perovskite Solar Cells

For perovskite solar cells (PSCs), the surface traps of perovskite films have great influence on the charge carrier behavior at the interface of perovskite and charge transport layers. In this investigation, a 248 nm KrF excimer laser with high photon energy and shallow penetration depth is introduced to perform surface modification on the CH3NH3PbI3 film through irradiation for reducing its surface trap density for the first time. A whole excimer laser surface modification (ELSM) process can be completed in few seconds, and the actual interaction time of the excimer laser and perovskite film is only several hundred nanoseconds. After ELSM, the trap density of the CH3NH3PbI3 film decreases from 1.61 × 1016 cm−3 to 5.81 × 1015 cm−3, and the nonradiative recombination is suppressed effectively. As a result, the open‐circuit voltage and the power conversion efficiency (PCE) of CH3NH3PbI3‐based PSCs increase from 1082 ± 27 to 1117 ± 16 mV and from 16.69% ± 0.77% to 18.50% ± 0.65%, respectively, and the PCE of the champion device reaches 19.38%. In addition, the measured larger charge recombination resistance, slower open‐circuit photovoltage decay, and longer charge recombination lifetime confirm the effective suppression effect of ELSM on the charge carrier recombination in PSCs.

Laser transmission welding and surface modification of graphene film for flexible supercapacitor applications

When a graphene film is coated onto a flexible polymer substrate, weak adhesion can cause delamination of the film under mechanical bending. Moreover, as each graphene layer restacks, the performance of the film as an electrode for a supercapacitor becomes limited. In this study, facile laser welding and surface modification processes are demonstrated to overcome these limitations. First, a continuous wave laser beam is applied to the interface between the coated graphene and the underlying transparent polycarbonate substrate. This welding process significantly improves their adhesion and enables excellent mechanical bendability. Second, surface modification of graphene is achieved under ambient conditions by irradiating the graphene film surface with a nanosecond pulsed laser. Sandwich-type supercapacitors are fabricated using these surface-modified graphene electrodes with a PVA-H3PO4 electrolyte. The effect of the laser fluence on the performance of the supercapacitor is investigated. At an optimal laser power, an areal capacitance of 4.7 mF/cm2 is achieved.

Suppression of shrinkage porosity in laser-joining of CFRP and steel using a laser surface modification process “Surfi-Sculpt ®”

The emergence of shrinkage porosity is a significant issue in the laser joining of carbon fiber reinforced polymer (CFRP) and steel. The joining process uses laser heating to melt a thermoplastic CFRP matrix in contact with a metal substrate to create a physical bond upon cooling. During the cooling process, melted CFRP at different location has different solidification rate, leading to the shrinkage porosity in the CFRP. In this study, the laser Surfi-Sculpt® is used to prevent the formation of shrinkage porosity in CFRP. The effects of the morphology characteristics (protrusion height and density) on shrinkage porosity are investigated, and the thermal distribution and history during the joining is simulated to examine mechanism of shrinkage porosity suppression. The results show that formation of shrinkage porosity can be suppressed with the laser Surfi-Sculpt®, which is attributed to the change of heat conduction path. The protrusions conduct the heat from the interior of melted zone to steel, decreasing the heat accumulation at the interface and suppressing the formation of shrinkage porosity. After adopting this technique, the porosity can be reduced from 7.13% to 1.26% and the shear strength of the joint increases from 9.3 MPa to 30.8 MPa.

A study of laser surface modification of polymers: A comparison in air and water

Laser surface modification is a technique to modify polymer surfaces for various applications. In our earlier work [Physics Procedia, 83:211–217, 2016], we showed that when the laser surface modification process was carried out in water instead of air, the obtained surface characteristics were remarkably different, which led to a significant improvement in the metal deposition characteristics using electroless plating. In this work, we try to explain the underlying fundamental mechanisms that contribute to this improvement in surface characteristics through concurrent experimental and modeling research. The observed images of laser modified surfaces suggest that a hemispherical hump is formed in the case of water at lower laser fluences that breakup with an increase in fluence. Such a behavior was not observed when the process was carried out in air. We explain this phenomenon by simulating the temperature profiles in the polymer during the laser heating process in air and water. The results suggest that subsurface heating effects occur when the process is carried out in water. We further argue that this phenomenon is mainly responsible for the formation of the complex structure that was observed in our previous work.

Effect of laser parameters on surface roughness of laser modified tool steel after thermal cyclic loading

This study presents the effects of laser parameters on the surface roughness of laser modified tool steel after thermal cyclic loading. Pulse mode Nd:YAG laser was used to perform the laser surface modification process on AISI H13 tool steel samples. Samples were then treated with thermal cyclic loading experiments which involved alternate immersion in molten aluminium (800°C) and water (27°C) for 553 cycles. A full factorial design of experiment (DOE) was developed to perform the investigation. Factors for the DOE are the laser parameter namely overlap rate (η), pulse repetition frequency (fPRF) and peak power (Ppeak) while the response is the surface roughness after thermal cyclic loading. Results indicate the surface roughness of the laser modified surface after thermal cyclic loading is significantly affected by laser parameter settings.

Compound technology of manufacturing and multiple laser peening on microstructure and fatigue life of dual-phase spring steel

The present work proposes an advanced double quenching and tempering heat treatment based laser surface modification process of dual-phase spring steel. Multiple laser peening without coating process utilized the decarburized surface as the protective layer for the further cold working process. The electron backscattering diffraction analysis on crystallographic orientation of individual grains and phase map exhibits a perfect dual-phase steel. Also, the high resolution transmission electron microscopic study explains the high strain induced microstructural grain refinement features and plastic deformation behaviors. The laser peening technique taking an advantage that it induces a large and high magnitude compressive residual stress with good thermal stability. The micro and nano-hardness profile provides better surface and sub-surface mechanical properties. The controlled average surface roughness is achieved in this course of work. The stress-strain characteristics on tensile properties are analyzed through the pre-fatigued specimens. The fully reversed high cycle fatigue test indicates that the current laser peening has substantially improves the fatigue life of the specimens.

An Investigation of Phase Crystallinity in Laser Modified Yttria Stabilized Zirconia (YSZ) Thermal Barrier Coating<sup></sup>

This paper presents laser surface modification process of plasma sprayed yttria stabilized zirconia (YSZ) thermal barrier coating (TBC) for enhanced hardness properties and low surface roughness. A 300W JK300HPS Nd: YAG laser was used to process YSZ TBC sample surface. The parameters selected for examination were laser power, pulse repetition frequency (PRF) and residence time. Micrographs of the TBC system were captured using EVO 15 Scanning Electron Microscope (SEM). Surface roughness was measured using 2-dimensional stylus profilometer. X-ray diffraction analysis (XRD) was conducted to measure phase crystallinity of the laser-modified coating surface. X-ray diffraction patterns were recorded in the 2θ range of 10 to 80° using Bruker D8 Advance system with 0.7Å wavelength from a copper source (~1.5Å). The laser modified surface exhibited higher crystallinity compared to the as-sprayed samples. The presence of tetragonal phase was detected in the as-sprayed and laser processed samples. The hardness properties of laser modified TBC increased 15% of the as-sprayed sample. These finding are significant to development of thermal barrier coating design optimization for enhanced surface properties of semi-solid forming die.

Atomic diffusion in laser surface modified AISI H13 steel

This paper presents a laser surface modification process of AISI H13 steel using 0.09 and 0.4 mm of laser spot sizes with an aim to increase surface hardness and investigate elements diffusion in laser modified surface. A Rofin DC-015 diffusion-cooled CO2 slab laser was used to process AISI H13 steel samples. Samples of 10 mm diameter were sectioned to 100 mm length in order to process a predefined circumferential area. The parameters selected for examination were laser peak power, pulse repetition frequency (PRF), and overlap percentage. The hardness properties were tested at 981 mN force. Metallographic study and energy dispersive X-ray spectroscopy (EDXS) were performed to observe presence of elements and their distribution in the sample surface. Maximum hardness achieved in the modified surface was 1017 HV0.1. Change of elements composition in the modified layer region was detected in the laser modified samples. Diffusion possibly occurred for C, Cr, Cu, Ni, and S elements. The potential found for increase in surface hardness represents an important method to sustain tooling life. The EDXS findings signify understanding of processing parameters effect on the modified surface composition.

An investigation of phase transformation and crystallinity in laser surface modified H13 steel

This paper presents a laser surface modification process of AISI H13 tool steel using 0.09, 0.2 and 0.4 mm size of laser spot with an aim to increase hardness properties. A Rofin DC-015 diffusion-cooled CO2 slab laser was used to process AISI H13 tool steel samples. Samples of 10 mm diameter were sectioned to 100 mm length in order to process a predefined circumferential area. The parameters selected for examination were laser peak power, overlap percentage and pulse repetition frequency (PRF). X-ray diffraction analysis (XRD) was conducted to measure crystallinity of the laser-modified surface. X-ray diffraction patterns of the samples were recorded using a Bruker D8 XRD system with Cu K α (λ=1.5405 A) radiation. The diffraction patterns were recorded in the 2θ range of 20 to 80°. The hardness properties were tested at 981 mN force. The laser-modified surface exhibited reduced crystallinity compared to the un-processed samples. The presence of martensitic phase was detected in the samples processed using 0.4 mm spot size. Though there was reduced crystallinity, a high hardness was measured in the laser-modified surface. Hardness was increased more than 2.5 times compared to the as-received samples. These findings reveal the phase source of the hardening mechanism and grain composition in the laser-modified surface.

Laser transformation hardening on rod-shaped carbon steel by Gaussian beam

Laser transformation hardening(LTH) is one of the laser surface modification processes. The surface hardening of rod-shaped carbon steel (SM45C) was performed by lathe-based laser composite processor with Gaussian-beam optical head. The LTH characteristics by dominant processes, longitudinal and depth directional hardness distributions and behaviors of phase transformation in hardened zones were examined. Especially, two concepts of circumferential speed and theoretical overlap rate were applied. When laser power increased or circumferential speed decreased, the surface hardening depth gradually increases due to the increased heat input. Moreover, the longitudinal hardness distribution particularly shows periodicity of repetitive increase and decrease, which results from tempering effect by overlap. Finally, the feasibility of laser transformation hardening is verified by using the beam with Gaussian intensity distribution.

WEAR RESISTANCE IMPROVEMENT OF 41CR4 STEEL ALLOY BY LASER SURFACE HARDENING

In this paper an overview is given about laser surface modification process, which is developed especially with the aim of wear resistance improvement. Pulsed Laser was applied to 41Cr4 steel alloy specimens using a Nd: glass laser. Laser processing parameters examined in the present study included power density (3*10^5 to 9.5*10^5 W/cm²). The power density and interaction distance was chosen such that the maximum temperature approaches the melting temperature. The importance of sufficiently high temperatures has already been recognized to reduce the wear rate at the highest power density. The 41Cr4 steel alloy samples were examined using optical microscopic inspection, microhardness test to determine the metallurgical phases and pin-on disc machine was used to estimate wear resistance. It was found in this study the microstructure were relatively dependent of laser power density. However, the laser power density had a significant effect on structural and mechanical properties especially wear resistance in a given power density level

고출력 다이오드 레이저(HPDL)를 이용한 탄소강 환봉의 표면변태경화

The laser material processing has replaced a conventional material processing such as a welding, cutting, drilling and surface modification and so on. LTH(Laser Transformation Hardening) is one branch of the laser surface modification process. A lot of energy is needed for the LTH process to elevate workpiece surface to temperature of the austenite transformation(A₃), which results from utilizing a beam with a larger size and lower power intensity comparatively. The absorptivity of the laser energy with respect to material depends on the wave length of a beam. This study is related to the surface hardening for the rod-shaped carbon steel by the high power diode laser(HPDL) whose beam absorptivity is better than conventional types of lasers such as CO₂ or Nd:YAG laser. Because a beam proceeds on the rotating specimen, the pretreated hardened-phase can be tempered and softened by the overlapping between hardened tracks. Accordingly, the longitudinal hardness measurement and observation of the micro structure was carried out for an assessment of the hardening characteristics. In addition, a hardening characteristics as a hardenability of materials was compared in the point of view of the hardness distribution and hardening depth and width.

탄소강 환봉의 레이저 표면변태경화 특성에 관한 연구 (I) - 가우시안 파워밀도 분포의 레이저 열원을 이용한 표면변태경화 특성 -

Laser Material Processing has been replaced the conventional machining systems - cutting, drilling, welding and surface modification and so on. Especially, LTH(Laser Transformation Hardening) process is one branch of the laser surface modification process. Conventionally, some techniques like a gas carburizing and nitriding as well as induction and torch heating have been used to harden the carbon steels. But these methods not only request post-machining resulted from a deformation but also have complex processing procedures. Besides, LTH process has some merits as : 1. It is easy to control the case depth because of output(laser power) adjustability. 2. It is able to harden the localized and complicated a.ea and minimize a deformation due to a unique property of a localized heat source. 3. An additional cooling medium is not required due to self quenching. 4. A prominent hardening results can be obtained. This study is related to the surface hardening of the rod-shaped carbon steel applied to the lathe based complex processing mechanism, a basic behavior of surface hardening, hardness distribution and structural characteristics in the hardened zone.